Overview
Huanglongbing (HLB) is a bacterial disease. The most effective way to treat bacterial diseases is by using antibiotics. Foliar applications of antibiotics to mitigate HLB have been tried but were ineffective because i) they do not penetrate the leaf cuticle effectively, and ii) the bacteria reside in the vascular system (i.e., the phloem) and are therefore distributed systemically throughout the tree. This means that the bacteria populate not only leaves, but also fruit, twigs, trunk, and roots.
Trunk injection is the targeted, systemic delivery of pesticides, nutrients, or other crop protection materials into the stem or trunk of woody plants as an alternative to spraying or soil drenching. In contrast to foliar sprays, trunk injection delivers chemical directly into the vascular system.
The plant vascular system contains two parts.
1) The xylem, in which water and dissolved nutrients are transported.
2) The phloem, in which carbohydrates and other small organic molecules are transported.
Injection occurs into the xylem (the wood of the trunk) from where the injected material is systemically distributed throughout the tree with the transpiration stream. This mode of delivery provides several advantages.
1) Materials are applied precisely and spray drift is eliminated.
2) Non-targeted organisms are less affected.
3) There is less risk for human health and the environment.
Numerous injection devices are currently available – most require the drilling of a hole into the trunk followed by the injection of the desired material using a pressurized injection device.
In the trunk, the phloem (where the HLB-associated bacteria reside) is a very narrow strip of tissue in the inner bark. Because injections are made into the xylem, any injected material directed against the HLB-associated bacteria must be mobile in both the xylem and the phloem. The materials must also be able to translocate from the xylem to the phloem following the injection. The mobility of a chemical throughout the tree vascular system depends on its chemical properties. Materials that are not phloem mobile will not be effective against HLB.
Numerous research studies by UF/IFAS have demonstrated the effectiveness of trunk injection delivery of antibiotics in improving tree health and productivity and enhancing fruit and juice quality. The technology has since been widely adopted by Florida citrus growers and was shown to be economically feasible when used on a commercial scale. Although generally positive, the effects of trunk injection therapy can vary based on various factors, including age and disease state of the trees, the growing environment, and the way it is applied.
